1. Field of the Invention
The present invention relates generally to high-power density Litz cables and, more particularly, is concerned with a connector assembly for transferring high-frequency current into an internally-cooled Litz-wire cable.
2. Description of the Prior Art
Under high-frequency excitation, an electrical conductor's current density ceases to be uniform. The well-known "skin effect" causes current to move away from the center of the conductor and crowd into a layer just beneath the surface. The effect is compounded in a coil of many turns, wherein the self-fields of each conductor turn induce current density changes in adjacent turns. In order to lessen the impact of the skin effect, standard power cables employ multiple strands of conductors whose cross section is significantly less than that of one larger conductor of the same total area. However, with increasing frequency the impedance (resistance and inductance) of the stranded cable increases because of current crowding caused by unequal magnetic flux linkages among the individual wires.
In contrast, a cable may be formed by transposing individual wires within small groups of wires and then transposing the groups within the conductor. The immediate effect of this cabling method is to equalize the flux linkages of each individual strand, thus causing the current to divide evenly among the strands. If the conductor is constructed loosely, ohmic heating is more evenly distributed in the conductor volume, allowing more efficient heat removal as compared to sheet, ribbon, solid, or hollow conductors. This concept of using individually transposed and insulated strands in a cable-like conductor dates back to the early days of radio engineering practice.
Transposed stranded wire conductors are commercially available under the name Litzendrant conductor or Litz wire. Typically, there are 5 to 19 strands within a group. Groups are then bunched together, typically 5 to 7 in a bunch. Then, bunches are ganged together, typically 4 to 7 in a cable. If more wires are needed for large, high-power cables, several cables are assembled and wrapped by exterior insulation. Each of these operations involves a helical twisting of the elements within the group. The "lay" of the wire can be tight or loose, depending on the pitch of the helical transposition. This has a major effect on the conductor's resistance to internal fluid flow.
For high-power density applications, such as the electromagnetic valve or flow control device disclosed in the above cross-referenced application, internal forced-air cooling of the conductor is required. Environmental factors in the valve design make the use of water cooling hazardous; therefore, air or liquid freon must be used Running at temperatures of 300 degrees C., the cooling requirements are so severe that high-pressure air (12 to 15 atm.) is required with flow rates of up to 5 cfm. This cooling air must be passed to and from the cable through a connector to ensure uniform air flow within the cable. If freon is used, the pressures range from 15 to 22 atm. (to keep the freon liquid) and temperatures of approximately 200 degrees F are reached at the cable outlet.
Thus, the overall requirements of passing high current through many (2,000 or more) wires at high frequency (10 kHz) with internal cooling presents a unique problem to the connector design. Consequently, there is a pressing need for a solution to this connector design problem.